Abstract:A large scale haze process that happened on Feb.5, 2007 over middle and eastern China was analyzed based on the air pollution index, conventional observation d ata, and NCEP reanalyzed data. The results show that both the forward tilling t rough and the temperature inversion are favorable condition for the haze formati on. The absolute values of vorticity, divergence and vertical velocity below 800 hPa are all quite smaller, which favorites to the persistence of haze. The varia tio n of air mixing height is a good indication to the formation and disappearance o f the haze. The mixing layer height appears higher in North China than that in S outh China during the haze periods, which makes it easier to form heavy hazy wea ther in South China. In addition, it was found that the relative humidity is low er, the difference between air temperature and dew temperature is higher, the in tensity of air temperature inversion is weaker and the mixing layer height is hi gher in the haze periods than in the fog periods，which can provide some helpful clew for haze forecasting.